1. Field of the Invention
The invention relates generally to the field of semiconductor manufacturing, and more specifically to a polishing system having a carrier head with substrate presence sensing.
2. Related Art
A wafer carrier is a critical component of a polisher. The wafer carrier serves two main purposes. A first purpose is to transport a wafer to/from a load station and between each polishing process area. A second purpose is to press the wafer downward against a polishing pad using a backside pressure while the polish pad and the wafer carrier rotate at high speeds. The type of carrier determines how pressure is applied to the backside of the wafer. One type of carrier includes an internal wafer presence sensor to verify that a wafer is loaded onto the carrier.
FIG. 1 is a cross sectional view of a carrier head having a substrate sensing mechanism according to the prior art. Carrier head 10 includes a perforated plate 12, and a gimbal plate 14 disposed within retaining ring 16. An edge control ring 20 holds a membrane 22 across a bottom surface of perforated plate 12. The substrate sensing mechanism of carrier head 10 includes a plunger 24 disposed within a sensor venting port 50 of gimbal plate 14. Plunger 24 is resiliently held within the venting port by a weak spring 26 disposed between a top portion of plunger 24 and an encapsulated region defined by reference numeral 28. An oversized non-captured O-ring 30 is disposed between a flange portion of the plunger 24 and a top surface of gimbal plate 14, around the venting port 50. Pressure sensor 32 monitors a pressure within encapsulated region 28. Under normal operating conditions, encapsulated region 28 is either pressurized or vented.
Plunger 24 can move vertically within sensor venting port 50 between a lower most travel position and an upper most travel position. The lower most travel position is defined by a combination of the plunger flange, the oversized non-captured O-ring 30, and the top surface of the gimbal plate 14. When in the lower most travel position, a bottom portion of plunger 24 extends below a lower most surface of perforated plate 12 by a distance indicated by reference numeral 36. The upper most travel position is defined by a top surface of the plunger flange and a surface above the flange within encapsulated region 28. When in the upper most travel position, a top portion of the plunger 24 is moved a distance as indicated by reference numeral 34.
FIG. 2 is a top view of a substrate sensor venting port and an oversized non-captured O-ring according to the prior art. For example, a portion of gimbal plate 14 containing the substrate sensor venting port 50 is shown. The diameter of venting port 50 is slightly larger than a diameter of the plunger 24 to allow the plunger 24 to move within port 50. To provide for venting, venting arteries or channels 52 are disposed along an inner sidewall of port 50, extending from a top surface of gimbal plate 14 to a bottom surface of gimbal plate 14. The use of the oversized non-captured O-ring 30 increases a possibility for impeding the venting of the encapsulated region, resulting in an erroneous sensing performance. That is, O-ring 30 is subject to various placements about the venting port 50, for example, off-center from the venting port 50. It is also possible for the placement of O-ring 30 to preclude passage of vacuum or pressure through one or more arteries 52.
Carrier head 10 suffers from reliability issues of the wafer sensing mechanism. Such reliability issues lead to various handling problems that include one or more of dechuck errors, false wafer loss alarms, and failure to detect wafer loss. A dechuck error generally refers to a situation wherein a wafer slips off the carrier onto the underlying polishing pad as the carrier attempts to lift off the polishing pad after processing, typically resulting in breakage of the wafer. A false wafer loss alarm generally refers to a situation wherein the carrier incorrectly senses no wafer presence although a wafer is physically loaded, typically resulting in various handling errors. A failure to detect wafer loss generally refers to a situation wherein the carrier incorrectly senses a wafer when a wafer is not physically present, typically resulting in wafer breakage of a wafer that gets left behind. Such problems cause product scrap, tool downtime/reduced availability, and increased wafer polishing and carrier consumable cost.
Accordingly, it would be desirable to provide a carrier head with improved wafer sensing to overcome the problems in the art.